Influenza vaccinations can be administered in primarily two different forms: one form with a live, but critically weakened virus (a.k.a. live attenuated vaccine); the other with a completely inactivated virus. Vaccines are also described by the number of different strains of the flu virus they protect against. Historically, the flu vaccine has protected against 3 different flu strains (trivalent vaccine). During the flu season of 2013-2014, a new variation of the flu vaccine was introduced in the United States. This new quadrivalent vaccine provides protection against four different potential seasonal strains of influenza virus. It is generally recommended that any children above the age of 6 months should receive an annual flu vaccination.

In this study, immunization records were analyzed at several sites in Michigan, Minnesota, North Dakota, New York City, Oregon, and Wisconsin in order to assess how the quadrivalent vaccine is being administered compared to the trivalent. At these sites, close to 2 million vaccines were administered to children aged 6 months to 18 years during the 2013-2014 season. Of these vaccinations, 49.8% were inactivated virus trivalent vaccines, 16.4% were inactivated virus quadrivalent vaccines, 30.1% were critically weakened virus quadrivalent vaccines, and 3.7% were unknown. The inactivated virus trivalent vaccine was the most common in every age group except for 5-8 year olds.

Since this was the first flu season when quadrivalent vaccines had been used, they were not readily available at all of the sites examined. This might help to explain why they were not used even more commonly.

Starting in 2014, the Advisory Committee on Immunization Practices has begun to recommend that healthy children, aged 2-8 years, should preferentially receive the critically weakened virus quadrivalent vaccine. This recommendation, along with increased availability of the new quadrivalent vaccines should increase the percentage of children receiving quadrivalent vaccinations in the future.

Although the sites examined in this study may not be a perfect representation of the entire country, they do provide a baseline to begin examining how quickly the quadrivalent vaccines are adopted in the United States.